In many applications, e.g., in mobile radio systems, it is necessary to synthesize frequencies in a digital way, i.e., using a Phase-Locked Loop. A conventional integer-N frequency synthesizer produces an output frequency that is N times the input reference frequency, such that its frequency resolution is the same as the PLL reference frequency. Therefore, narrow channel spacing is accompanied by a small loop bandwidth, which leads to long settling times. With a fractional-N frequency synthesizer, an output frequency is generated that is N+X/Y times the input reference frequency, i.e., a fractional multiple of the reference frequency, such that narrow channel spacing is achieved along with a higher phase detector frequency. If Y is not too big the fractional-N frequency synthesizer can be based on multiphase clock signals. The Voltage Controlled Oscillator (VCO) then disposes of Y copies of the signal, each shifted over 2π/Y. The value of X then determines at which instances a VCO output pulse is generated.
Several major drawbacks arise from this approach. A mismatch between the various clock signal phases causes reduced quadrature accuracy, if the phases are used in an image-reject transceiver. Further, when the PLL is locked, the delay mismatches introduce periodic phase errors that give rise to fractional spurs in the output frequency spectrum, resulting in an out-of-spec transmitter spectrum and in a reduced interference capability in the receiver. A solution to this problem is suggested in IEEE JSSC, Vol. 36, No. 5, May 2001, pp. 777-783. It consists in adding to the PLL a self-calibrating loop to eliminate the delay mismatches. The calibration loop adjusts the phases of the multiphase clock signal based on the timing information present in the phase frequency detector (PFD) outputs. The calibration loop has a much smaller bandwidth in order to avoid disturbance of the locking behavior of the main loop. A safe solution here is to activate the calibration loop only when the main loop is locked. In the calibration loop there is a multiplexing switch that guides the current coming out of the calibration charge pump towards one of the Y calibration loop filters. Which one of the Y calibration loop filters is to be selected is determined by a control logic that knows which phase is currently selected by the phase-switching fractional divider and thus knows which phase must be calibrated.
To adjust the phase of the multiphase clocks, extra circuitry must be inserted in the VCO or in the high-speed divide-by-2 prescalers. This inevitably slows down these circuits, resulting in lower maximum operating frequency and/or higher power consumption.